The production of a fast spun polyester filament yarn consisting of at least 85 weight % of ethyleneterephthalate units, leads to a feeder-yarn which, because of its highly oriented amorphous qualities, is eminently suitable for false twist texturing. However, these properties can only be obtained if fast cooling can be effected at high winding up speeds.
Basically, two different methods were proposed to obtain such a shock cooling process.
Thus EP-A-O 089 819 describes a shock cooling process by means of water. In this process, a polyethyleneterephthalate filament yarn is cooled in a water bath at a rate of at least 5000 m/min. While being highly oriented, the resultant yarn only has a low crystallinity, substantiated by a boiling shrinkage of at least 45% and at most 68.5%.
However, cooling in water has serious drawbacks. First of all, the yarn is subjected to breaking in the water bath, resulting in high stresses. There occur problems with spraying water and the application of a spinning preparation onto the wet yarn. When using water cooling, there occurs a rapid increase in the water carried along adhering to the surface, and hence in the spraying water, as the number of filaments increases.
As compared with shock cooling with water, an improvement has been obtained with air cooling. Thus EP-A-O 244 216 describes a polyester filament yarn which is spun at a rate of over 5000 m/min in a high pressure chamber where a narrow tube is arranged at its outlet, shock cooling being obtained because of the Venturi effect. In the case of air cooling, the uniform cooling of the gathered filament bundle in the cooling tube (Venturi) is no longer ensured as the number of filaments increases.
Apart from the drawback of the high specific air consumption of up to approximately 70 Nm.sup.3 /kg of polyethyleneterephthalate, it is not possible to ensure a defined yarn speed in the range below the spinneret, exactly at the point where the most intensive cooling must be effected.